Recently, according to the technology development, liquefied gas, such as liquefied natural gas and a liquefied petroleum gas, is widely used, substituting gasoline or diesel.
The liquefied natural gas is obtained by cooling and liquefying methane obtained by refining natural gas collected from a gas field, and is colorless and transparent liquid, has little pollutants, and has high calorie, so that the liquefied natural gas is very excellent fuel. In the meantime, the liquefied petroleum gas is fuel obtained by compressing gas which has propane (C3H8) and butane (C4H10) collected from an oilfield together with petroleum as main components at a normal temperature and making the compressed gas into liquid. The liquefied petroleum gas is colorless and odorless, like the liquefied natural gas, and is widely used as fuel for household, business use, industrial use, a car, and the like.
The liquefied gas is stored in a liquefied gas storage tank installed on the ground or a liquefied gas storage tank provided in a vessel that is a transport means voyaging the sea, the liquefied natural gas is decreased to 1/600 in volume, and propane of the liquefied petroleum gas is decreased to 1/260 in volume and butane thereof is decreased to 1/230 in volume, so that the liquefied gas has an advantage in high storage efficiency. A temperature, pressure, and the like required for driving an engine using the liquefied gas as fuel may be different from a state of the liquefied gas stored in the tank.
Further, when the LNG is stored in a liquid state, heat penetrates to the tank, so that some of the LNG is vaporized and Boil Off Gas (BOG) is generated, and the BOG may cause a problem to a liquefied gas processing system. Because of this, the related art attempts to solve the problem by consuming the BOG by a method of discharging the BOG to the outside and burning the BOG (in the related art, in order to remove a risk in damage to the tank by decreasing pressure of the tank, the BOG is simply discharged to the outside and is processed), but the method causes environment contamination and resource waste.
Recently, as a technology for efficiently processing BOG, a utilization method of re-liquefying the generated BOG and supplying the BOG to an engine and the like is implemented, but despite of the utilization, the BOG is not sufficiently consumed, so that it fails to efficiently utilize a resource.
Ship owners propel vessels by using an MEGI engine that uses LNG as fuel to excellently and effectively respond to the recently carried NOx discharge regulation and environment contamination prevention. However, the MEGI engine has problems in that pressure required for driving an engine is 300 bars which are very high, so that power consumption is huge, a considerable amount of installation cost is required, and a configuration of the system is complex to require a large installation area.
Accordingly, research on an engine which is capable of replacing the MEGI engine is conducted to develop a low-speed two-stroke log pressure injection engine (2sDF or XDF), and a need to development of a fuel supply system using the low-speed two-stroke log pressure injection engine is on the rise.